Polymorphism, mesomorphism and anti-oxidant character of homoleptic bis[(E)-2-(1-((o-ethylphenyl)imino)ethyl)phenolato-κ2-N,O]Ni/Cu(ii) complexes

Abstract

The proligand (E)-2-(1-((o-ethylphenyl)imino)ethyl)phenol (HL) reacts with nickel(II) or copper(II) acetate to yield homoleptic bis[(E)-2-(1-((o-ethylphenyl)imino)ethyl)phenolato-κ²-N,O]nickel(II) (1) and copper(II) (2) complexes. Solid-state structures of both complexes were obtained, each bearing two equivalents of the N,O-chelating ligand and a square-planar geometry at the metal. The cupric species crystallized in two distinct habits – as blocks (2a) and rhomboids (2b). Despite the presence of different divalent metal ions, 1 and 2a are isostructural and both crystallize in the triclinic space group P, while polymorph 2b is monoclinic (space group P2₁/n). The bulk isolated material of 2 is demonstrated to contain polymorph 2b almost exclusively by powder X-ray diffraction. DFT modelling supports the relative stability of the two polymorphs. Differential scanning calorimetry (DSC) and polarized light microscopy (PLM) reveal that the cupric complex in morphology 2b has mesogenic properties. Cyclic voltammograms diagnose irreversible redox events for both complexes in DMF, while the Schiff base and complexes exhibit a moderate to medium level of anti-oxidant scavenging activity.